The prior art has recognized the need to perform air separation, particularly for the recovery of nitrogen with greater efficiency. With the increasing cost of energy and the need for large quantities of separated gas such as nitrogen for enhanced petroleum recovery, highly efficient separation processes and apparatus are necessary to provide competitive systems for the separation and production of the components of air, most particularly nitrogen.
The prior art has attempted to provide such efficiencies with various systems using the integration of process streams in an air separation plant, as well as various forms of autorefrigeration produced from the expansion of a high pressure stream in an expansion turbine or the flashing of a process stream through a JT valve.
In U.S. Pat. No. 2,627,731 a process for the rectification of air into oxygen and nitrogen is described wherein a two sectioned or single distillation column are used alternatively. Air is cooled by heat exchange and introduced directly into the distillation column. A nitrogen product is removed from the overhead of the column and a portion is compressed in two stages. The first stage nitrogen compressed stream is recycled in order to reboil and condense a portion of the midpoint of the column by indirect heat exchange before being introduced into the overhead of the column as reflux. A second stage compressed nitrogen stream is recycled and partially expanded to provide refrigeration. This expanded stream is recycled to the nitrogen product line. The remaining stream of the second stage compressed nitrogen stream reboils the bottom of the column before being combined with the first stage compressed nitrogen stream and introduced into the overhead of the column as reflux.
In U.S. Pat. No. 2,982,108, an oxygen producing air separation system is set forth wherein a portion of the nitrogen generated from the distillation column is compressed and reboils the base of a high pressure section of the column before being introduced as reflux to the low pressure section of the column. The feed air stream is supplied in separate substreams into the high pressure section of the column and in an expanded form into the low pressure section of the column.
U.S. Pat. No. 3,492,828 discloses a process for the production of oxygen and nitrogen from air wherein a nitrogen recycle stream is compressed and condensed in a reboiler in the base of a distillation column before being reintroduced into the column as reflux. A portion of the nitrogen recycle stream may be expanded in which the power provided by the expansion drives the compressor for the main nitrogen recycle stream.
In U.S. Pat. No. 3,736,762, a process for producing nitrogen in gaseous and liquefied form from air is set forth. A single distillation column is refluxed with nitrogen product condensed in an overhead condenser operated by the reboil of oxygen conveyed from the bottom of said column. At least a portion of the oxygen from the overhead condenser is expanded to produce refrigeration for the separation.
In U.S. Pat. No. 4,222,756, a process is set forth in FIG. 4 in which a two pressure distillation column is used in which both pressurized column sections are refluxed with an oxygen-enriched stream. The low pressure column is fed by a nitrogen-enriched stream from the high pressure column which is expanded to reduce its pressure and temperature.
U.S. Pat. No. 4,400,188, commonly assigned, discloses a nitrogen production process wherein a single nitrogen recycle stream refluxes a distillation column which is fed by a single air feed. Waste oxygen from the column is expanded to provide a portion of the necessary refrigeration.
Although the prior art has taught numerous systems for the separation of air and particularly the production of a nitrogen product from air, these systems have been unable to achieve the desired efficiencies in power consumption and product recovered which are necessary in the production of large volumes of air components, such as nitrogen. The present invention using significant integration of process streams achieves greater efficiency in the production of air components, such as nitrogen, particularly for large volumes of gaseous product.